OK, my wife is an INFJ, and my niece (age 16 3/4) is an INFP.
I know both very well, and will offer my commentary as to your inquiries.
The INFP and INFJ can a number of things in common, but what are the most telling differences between the two types?
In general, INFP is more "laid back" than INFJ.
From what I can tell, INFJ is more "formal" in dress/appearance, and INFP is more "casual."
Beside these two items I have seen as obvious differences, I would have to say that stark differences are going to exist more clearly when comparing two individuals...
How are they most different socially?
INFJ seems better at engaging in socialization with strangers, INFP seems better at managing on-going (established) social relations.
How are they most different intellectually?
INFJ = Good at science, writing, math to a degree, and physical fitness.
INFP = Great at art, music, writing, social studies, history, physical fitness is ancillary...unless we are talking about Biaxident - the biggest INFP in the WORLD!

umpyouup:
How are they most different emotionally?
Both very sweet.
Both prone to melancholia.
INFJ more prone to "shutting down" or "ghosting" (being in the same room, but not "present."), whereas INFP more prone to "letting it all hang out."
How would you contrast the way the INFJ vs. the INFP approaches information and learning?
INFJ is more focused and directed.
INFP is more free-form, but still pursues the same level of mastery, albeit in a differnet way, more sporadic.
Which type is more stand-offish?
INFJ is more stand-offish when they do not "feel comfortable" or when they have a "conflict" with the emotional energy being exuded from a given person at a given time.
My wife is an open book; if she does not like you, you will know it.
My niece is more passive; if she does not like you, she will engage with you to the minimum degree possible, and then avoid you.
Which type connects more quickly with friends?
INFP is more quick to connect with established friends.
INFJ's connections with established friends are deep but seem to connect less frequently.
Which type is more reclusive?
INFP seems to be.
INFJ seems to engage in basic socialization a bit more, especially if it is part of participating in some activity that they "identify with."
INFP's don't seem to need to "identify" with things a whole lot.
Which type shows more emotion, which one recluses when processing emotion?
INFP shows emotion more frequently.
INFJ shows more emotion when it is expressed.
---------------------
In short, I will confess that I have inadvertantly had a life long fascination with Idyllics. Both of my major SO's have been xNFJ. When I got into typology, I tested a slew of my friends, and I was shocked at how many of my really good, close friends were Idyllics. They seem to value the happy go lucky lunatic in me, and my sense to be practical and a real-time problem solver. I have always enjoyed their dry, witty humor, and their capacity to always find a way to make things better. An Idyllic with a positive attitude is a joy to be around. ESTPs can at times be blockheaded and exceedingly blunt, and there is no one better than an INFx to let them know the error of their ways, without triggering feelings of wrath or rejection. I luvs ma' INFx's!
